Members of the epidermal growth factor receptor (EGFR) family play key roles in the control of cell growth, and thus when aberrantly activated, contribute to the development of a number of human cancers. Our long-term objectives have been to use a combination of biochemical, genetic, and structural biology-based approaches to determine how EGFR family members are regulated and the nature of the specific signaling pathways that they use to direct a variety of biological outcomes. During the course of these studies, we discovered a new end-point for growth factor-coupled signaling, that influences gene expression at the level of RNA processing and involves the regulated binding of N7-methylguanosine-capped RNA to the cap- binding protein complex (CBC). This target appears to be highly selective for signaling pathways involving the EGFR family member ErbB2/Neu. Thus, heregulin (HRG), a growth factor that stimulates the activation of ErbB2, is significantly more effective than EGF in regulating the CBC. During the past funding period, we have obtained clues regarding how HRG might use the Rho-related GTPases, Cdc42 and Rac, and their GEFs, the Cool (Cloned-out of Iibrary)/Pix (PAK-interactive exchange factors) proteins, in the initial steps of this pathway to signal downstream to the mammalian target of rapamycin (mTOR) and p70 S6 kinase. We also discovered that the nucleocytoplasmic transport proteins, a- and p-importin, as well as the Ran GTPase are key participants in this signaling pathway, acting further downstream by directly mediating the regulation of the CBC's cap-binding activity. In the coming project period, we will take advantage of these and other recent findings from our laboratory to address what we believe are fundamentally important questions regarding how growth factors signal to the nucleus and how their signaling pathways are designed with the specificity required for directing distinct cellular functions. The three specific aims of the proposed studies are as follows: 1) How does HRG signal to the Rho GTPases in the early stages of a signaling pathway that regulates RNA processing? We are particularly interested in understanding what makes HRG so effective in activating Cdc42 and Rac, as well as determining how the Cool/Pix GEFs promote their activation and whether this involves a Cdc42-Rac signaling cascade. 2.) How does HRG direct the regulation of mTOR? Here we will set out to determine how Cdc42/Rac mediates the growth factor-dependent regulation of mTOR and stimulation of p70 S6 kinase activity. 3.) Understanding how HRG directs the regulation of Ran and the cellular consequences of the growth factor-stimulated activation of Ran and the CBC. The expectation is that the studies proposed in this renewal application will highlight important new participants and regulatory mechanisms in receptor-coupled signaling and potentially identify new targets for therapeutic intervention against cancer and other diseases linked to EGFR family members.